Cyclopentenyl cytosine, one of a new series of nucleoside analogues, was found to have marked in vivo antitumor activity in several murine models. It was especially active against an Ara-C resistant L1210 tumor. Inhibition of CTP synthesis was confirmed in mice bearing L1210 ascites. Four new cyclopentenyl nucleosides were synthesized and evaluated. Two pyrimidine nucleoside analogues, cyclopentenyl thymine and cyclopentenyl Ara-C, showed no inhibition of L1210 cell growth at concentrations of 0.1 mM. Cyclopentenyl nebularine inhibited L1210 cell growth by 65% at 10 MuM and cyclopentenyl 8-aza- adenosine inhibited by 33% at 2 MuM and 73% at 5 MuM. A method was developed to quantitate dihydrolenperone, a cytotoxic agent with lung tumor specificity. This analysis is being used in a Phase I-II pharmacokinetics study of this agent.